1. Field of Invention
The present invention relates to a method of forming bumps. More particularly, the present invention relates to a method of forming high-quality bumps inside a high-density package.
2. Description of Related Art
In this information explosion age, integrated circuit products are used almost everywhere in our daily life. As fabricating technique continue to improve, electronic products having powerful functions, personalized performance and a higher degree of complexity are produced. Nowadays, most electronic products are relatively light and have a compact body. Hence, in semiconductor production, various types of high-density semiconductor packages have been developed. Flip chip is one of the most commonly used techniques for forming an integrated circuit package. In a flip-chip package, the bonding pads on a die and the contact points on a substrate are connected together through a plurality of bumps. Hence, compared with a wire-bonding package or a tape automated bonding (TAB) package, a flip-chip package uses a shorter electrical path on average and has a better overall electrical performance. Moreover, the back of a flip-chip die may be exposed to increase heat dissipation. Due to the above and other reasons, flip-chip packages are produced in large quantities in the semiconductor industry.
FIGS. 1 through 5 are partially magnified cross-sectional views showing the progression of steps in a conventional method of forming a bump on the surface of a chip. As shown in FIG. 1, a silicon wafer 110 is provided. The wafer 110 has an active surface 112 with a passivation layer 114 and a plurality of contact pads 116 (only one of the contacts is shown) thereon. The passivation layer 114 exposes the contact pads 116. An under-ball metallic (UBM) layer 120 is formed over the contact pad 116. The under-ball metallic layer 120 includes an adhesion layer 122 and one or a stack of metallic layers 124. To form the under-ball metallic layer 120, a sputtering process is first conducted to form an adhesion layer 122 on the active surface 112 of the wafer 110. Next, a sputtering or plating process is conducted to form one or more metallic layers 124 over the adhesion layer 122. Thereafter, photolithography and etching processes are used to pattern the under-ball metallic layer 120 so that a residual portion of the under-ball layer 120 remains on top of the contact pad 116.
As shown in FIG. 2, a spin-coating process is conducted to form a photoresist layer 130 over the active surface 112 of the wafer 110. Through photolithography and etching processes, a plurality of openings 132 (only one opening is shown) are formed in the photoresist layer 130. The openings 132 expose the under-ball metallic layer 120. A printing process is conducted to deposit solder material 140 into the opening 132 of the photoresist layer 130 as shown in FIG. 3. The solder material 140 includes granulated metallic particles, a reductant, a dispersion agent and other solvents. Through a reflex process, the metallic particles inside the solder material 140 are fused to form a hemispherical solder block 140 as shown in FIG. 4. The solder block 150 is formed directly over the under-ball metallic layer 120 while the reductant, the dispersion agent and other solvents move to the surface of the solder block 150. Thereafter, a liquid cleaner is applied to remove the reductant, the dispersion agent and other solvents from the surface of the solder block 150. Finally, the photoresist layer 130 is removed from the active surface 112 of the wafer 110 as shown in FIG. 5 so that a bump 160 is produced. The bump 160 actually comprises the solder block 150 and the under-ball metallic layer 120.
In the aforementioned fabrication process, the solder material 140 is deposited into the openings 132 of the photoresist layer 130 by printing. However, the printing process may result in the solder material 140 filled into part of the openings 132. Hence, dimensions of the solder blocks 150 after a reflow process may vary considerably. Also, if the dimensions of the openings 132 of the photoresist layer 130 vary over a range, the volume of solder material 140 deposited into the openings 132 varies considerably. Accordingly, the dimensions of the solder blocks 150 after a reflow process varu considerably due to the above reason.
Since the solder material 140 contains reductant, dispersion agent and other solvents, these materials may react with the photoresist layer 130 to produce gases that are trapped in the solder block 150. Alternatively, if some residual reductant, dispersion agent or other solvents remain within the solder block 150 without moving to the surface of the solder block 150, voids are created inside the solder block 150. These internal voids may compromise the reliability of the bond between the solder block 150 and the substrate (not shown).
Since the solder material 140 contains reductant, dispersion agent and solvents, besides metallic particles, the solder block 150 after a reflow process will have a volume considerably smaller than the solder material 140 deposited in the opening 132. Thus, to form a solder block 150 having the correct height level and volume, the photoresist layer 130 must have considerable thickness and the opening 132 of the photoresist layer 130 must have considerable dimension. Hence, the solder blocks 150 must be separated from each other by a considerable distance, rendering the production of higher-density packages difficult.
Furthermore, after printing the solder material 140 into the opening 132 of the photoresist layer 130, if the solder material 140 is suspended in the opening 132, the solder material 140 separated from the under-ball metallic layer 120, the disconnection between the solder material 140 and the under-ball metallic layer 120 is not easy to be discovered. Thus, a missing block phenomenon may occur after a reflow process.